Construction Joints In Slabs And Basement Walls

A construction joint is the line where one concrete pour stops and the next one starts, and it needs to sit where the engineer planned for it. In a suspended slab that usually means away from the columns, and in a basement wall it means a detail that stops water finding a way through.

Every building has them, because no crew pours a whole tower in one go. The real question is whether each joint was designed, or whether it simply happened where the last truck ran out.

Why a joint needs designing

Concrete poured against hardened concrete doesn't bond the way a single pour does. The joint is a weaker plane through the member, and if it lands in the wrong place, it lands exactly where the shear or the water pressure is highest.

AS 3600 sets out how shear crosses a surface between old and new concrete, and the roughness of that surface is part of the calculation. So the position of the joint and the way it is prepared are both design decisions, even when the builder is the one proposing them.

Where joints go in a suspended slab

Shear is highest near the columns and walls, which is also where a flat slab is already working hard against [punching shear](/blog/punching-shear-in-flat-slabs). A joint belongs away from those zones, nearer the middle of a span, where shear is low.

The top and bottom reinforcement runs straight through the joint, and only the concrete stops.

Pour size comes next. A floor plate gets split into pours to suit the crew, the pump and the day's concrete supply, and I would rather draw those breaks with the builder at the start than find them marked in spray paint on the deck.

Post-tensioned slabs

In a [post-tensioned slab](/blog/post-tensioned-concrete-when-and-why), a pour break is also a stressing question. Tendons either anchor and get stressed at the joint, or pass through it to be stressed once the next pour is in, and that sequence goes on the PT drawings.

Large PT floors also get a pour strip, a narrow band left open between two pours. The concrete either side shortens as it is stressed and as it dries, and the strip is filled once enough of that movement has happened. Close it too early and the slab cracks against the columns and walls holding it.

The joint between a basement slab and wall

It is a common place for a basement to leak. Water under pressure looks for the weakest line, and a cold joint at the base of a wall is exactly that.

On a basement that has to keep water out, the joint gets a waterstop. It can be a PVC strip cast across the joint, or a hydrophilic strip that swells when it gets wet, and either one has to stay in position while the concrete goes in around it. A short upstand called a kicker, cast with the slab, lifts the joint above the floor and gives the wall formwork something firm to sit against. The wider waterproofing picture is in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

Vertical joints and pour lengths

A long wall cast onto a footing that has already hardened can't shrink freely, because the base holds it back. That restraint pulls the wall apart in vertical cracks, and in a basement those cracks carry water.

So walls are poured in planned lengths, with vertical joints between them and the waterstop detail carried through each one. Long slabs on ground follow the same thinking, set out in [expansion and control joints in concrete](/blog/expansion-and-control-joints-in-concrete).

Surface preparation

The first pour has to be ready for the second. Laitance, the weak, milky skin left on top of fresh concrete, comes off, and the surface is roughened and cleaned so the new concrete can grip it. Expanded metal mesh is a common stop end, because it leaves a rough, keyed face behind it.

Loose stones, sawdust, tie wire offcuts and water lying in the joint all come out before the next pour.

When the pour has to stop early

Sometimes the day goes wrong. A pump breaks down, the weather turns or the plant can't keep up, and the pour has to stop somewhere nobody planned.

That is a phone call to the engineer, made while the concrete is still workable. We can tell the builder where to stop and what to add, whether that is extra bars across the joint or a waterstop that wasn't in the original pour. A call made after the concrete has gone off turns a quick answer into a remediation design.

How we approach it

We draw the pour sequence on the structural drawings, with the joint positions, the waterstop details and the stressing sequence for PT floors. During construction, our pre-pour inspections check the joint as closely as the reinforcement, because it is the part of today's pour that tomorrow's pour depends on.

A joint is the one place where yesterday's concrete has to trust today's.

ACSES provides structural engineering, including construction supervision, across Sydney. Talk to us about a project.

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